In any electronic postage meter it is important that the electronic registers faithfully reflect the actual mechanical printing of postage by the print rotor. In nearly all instances of the printing of postage, electric power is available for the entirety of the print cycle and the processor, which controls and monitors printing of postage, is able to decrement and increment the descending and ascending registers accordingly.
Most of the time, when the rotor is not in motion (because it is not printing postage) the rotor is in or very near to a "home" position. It is standard to have a sensor that indicates to the processor that the rotor is in the home position. A typical sensor arrangement is to place a permanent magnet on a disk attached to the rotor shaft. A Hall-effect sensor is positioned so that the sensor is "on" when the rotor is in its home position. If a print cycle is in progress, the processor can confirm the completion of the cycle by sensing the return of the rotor to the home position.
It is also standard to have a ratchet or other mechanical arrangement coupled to the rotor shaft so that the rotor cannot be moved very far backwards. However, a number of design constraints limit how tightly the ratchet performs its task. For example, at the end of a print cycle, it is not uncommon that the rotor may reach the home position (as expected) but may also go slightly past the home position due to rotational inertia or due to drag from a large item being franked. If the meter is to be satisfactory to the user, the ratchet must permit the slight reverse rotation back to the home position.
The design of the meter must necessarily take into account, however, the prospect that power may fail unexpectedly. The processor may thus be in the situation of not having quite enough information to distinguish whether or not a print cycle has occurred. For example, one of the indications to the processor that a print cycle has begun is the home sensor turning off. This indicates that the rotor has moved from the home position. But if power is lost after the home sensor has turned off, then later when the processor again has power the processor may find that the home sensor is again on. This could have happened, however, due to either of two mechanical sequences.
On the one hand, the rotor may indeed have printed a print cycle, finished while processor power was absent. This would result in the home sensor again being on.
On the other hand, the rotor might merely have moved slightly past the home position, and then dropped backwards again to the home position, all while processor power was absent. This, too, would result in the home sensor again being on. This slight forward-and-back movement could, as described above, arise due to a slight overrun of a previous print cycle.
The difficulty, however, is that without more information the processor is not capable of distinguishing which mechanical sequence actually occurred. There is the difficulty, then that the meter designer would not know whether or not to program the processor to increment and decrement the ascending and descending registers in the face of such sensor inputs. This presents the possible problem that the amount of postage actually printed may fail to correspond perfectly with the information in the descending and ascending registers.
One prior-art approach to this problem is that found in U.S. Pat. No. 4,253,015 to McFiggans et al. ("the '015 patent"). As described in FIGS. 1 and 3 of that patent, there is provided a counter gear 61 that is twice as large as a gear 60 on the rotor shaft; the counter gear 61 has an angular velocity at all times half that of the rotor (also called a "drum" in this patent). Depending on whether the rotor has finished an even-numbered or odd-numbered printing cycle, the counter gear 61 will be in one of two orientations. Two holes 62 and 63, located at different radii on the counter gear 61, will trigger one of two sensors 67 or 66 respectively, permitting the processor 120 to know whether the print cycle just finished was an odd-numbered one or an even-numbered one.
This approach has a number of drawbacks, chief among them that it requires a large counter gear which takes up space in the meter, which adds to the count of moving parts, and which adds to the inertial moment that must accelerate and decelerate with each print cycle. The approach degrades the mechanical reliability of the meter, adds to the parts and assembly cost, and adds to wear due to use.
It would thus be desirable to have a way of providing sensor information to the processor to resolve ambiguities regarding rotor movement during times when the processor has been unpowered, all without unduly degrading reliability and increasing parts and assembly cost.